Filter vial

ABSTRACT

A filter vial and piston are provided where the vial has a cylindrical wall with a closed bottom and open top and with the hollow, tubular piston therein. The piston has a distal end covered by a cup having a proximal cup seal extending outward to engage the walls of the vial to form a fluid tight seal with the vial during use. The distal end of the piston has a piston flange extending outwardly and located adjacent a recess in the piston so the cup forms a snap fit over the piston flange and extends into the recess. The piston flange causes the cup to bulge sufficiently outward to form a fluid tight seal with the vial during use. A filter is placed in the bottom of the cup and abuts an inwardly extending lip on the cup. The filter is held against the lip by a piston support at the distal end of the piston. A cup support at a distal end of the cup also helps support the filter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of patent application Ser. No.12/856,324 filed Aug. 13, 2012, which is a continuation of patentapplication Ser. No. 12/053,241, now U.S. Pat. No. 7,790,117, filed Mar.21, 2008, the contents of which are incorporated herein by reference.

STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT

Not Applicable

BACKGROUND

Filter vials have a tubular piston with a filter at the distal end ofthe piston. The piston fits into a mating vial containing a fluid andthings suspended in the fluid. When the piston is inserted into the vialfluid passes through the filter and into the piston in order to separatethe fluid from particles or molecules too large to pass through thefilter in the bottom of the piston. The filtered fluid can be extractedfrom the piston for further use.

The diameter and thickness of the filters can vary greatly from filterto filter and the thickness of the filter can vary greatly even acrossone filter. These variations make it difficult to seal the filters soall the fluid being filtered passes through the filter, and so thatfluid does not wick around the peripheral edge of the filter tocontaminate the filtered fluid in the piston. Further, as fluid isforced through the filters the filters can bow or otherwise deform andallow fluid to bypass the filter as it enters the body of the piston.Current assemblies use ultrasonically welded assemblies. Even if thefilters are ultrasonically welded to the adjacent walls that leavesplasticizers that contaminate the fluids placed in the vials during use.

BRIEF SUMMARY

A filter vial and piston are provided that has a cup which holds afilter in position on the piston during use so that all the fluid passesthrough the filter and none passes around the edges of the filter. Theparts are integrally molded so there is no contamination from ultrasonicwelding. The parts snap-fit together, but appropriates seals areprovided to prevent wicking or leakage at the location of thesnap-locks.

The filter vial assembly has a longitudinal axis along which the pistonmoves during use. The assembly includes a tubular vial having a closedbottom and an open top with walls defining a cylindrical interior havinga diameter D. A tubular piston is provided having opposing distal andproximal ends with at least the distal end being open. The piston haswalls defining a cylindrical interior with an exterior facing recessedportion adjacent the distal end the piston. The piston further has adistal piston flange at or adjacent to the distal end of the piston andextending outward from piston. A filter support is located inside thepiston at the distal end of the piston. A retainer cup is provided,having a sidewall extending between a proximal and distal ends of thecup with an interior diameter smaller than the distal seal on thepiston. The cup has a lip extending inward toward the longitudinal axisat the distal end of the retainer cup, and further has a cup sealextending outward from the proximal end of the cup a distance greaterthan the diameter D. The sidewall of the cup are sufficiently flexibleto deform outward as the cup passes over the distal seal on the pistonso the sidewalls assume an outer dimension larger than diameter D at thelocation of the piston flange when the retainer cup passes over thepiston distal seal. A filter is interposed between the retaining cup'slip and the distal end of the piston when the proximal end of theretainer cup is placed over the distal end of the piston.

The piston and retainer cup are placed through the open end of thefilter vial so the filter cup abuts and forms a fluid seal with theinside of the vial at the location of the cup seal and at the locationof the distal piston flange. Preferably, the recessed area extendsentirely around a circumference of the piston. Further, while the lippreferably extends continuously around an inner circumference of theretainer cup, it could be intermittent. The piston, vial and retainingcup are made of plastic, and preferably each part is separately formed.As desired, a cap is provided where the cap is sized to releasably butsnugly fit over the proximal end of the piston. Such caps are commonlyused, and typically have a flexible membrane through which a needle canbe inserted to withdraw filter fluid from inside the piston.Advantageously, but optionally, the proximal cup seal abuts an end ofthe recess in the piston during use. Preferably, there is an additionalseal extending outward from the piston adjacent the proximal end of thepiston and located to engage and form a fluid tight seal against thevial when the distal end of the piston is adjacent the bottom of thevial. Ideally, the piston flange has a generally triangular crosssectional shape which is larger at the proximal end and smaller at thedistal end of the seal.

There is also provided a method of forming a piston for a filter vial,where the piston has a cylindrical tube with opposing proximal anddistal ends. The method includes forming an outwardly extending pistonflange at the distal end of the piston. A cylindrical recess is formedaround a periphery of the distal end of the piston at the distal end ofthe piston and adjacent to the piston flange and on the proximal side ofthe piston flange. A filter is placed in the bottom of a retaining cupwhich cup is sized to fit over the distal end of the piston. Theretaining cup is provided with an open proximal end and an inwardextending lip at an opposing distal end which lip abuts a periphery ofthe filter. The method also includes placing the open proximal end ofthe cup over the distal end of the piston and piston flange to form asnap fit between the proximal end of the cup and the recess on thepiston and advancing the retaining cup along the piston until the distalend of the piston abuts the filter. The retaining cup is provided withan outwardly extending seal at the proximal end sized to sealinglyengage the inside of the filter vial during use. The retaining cup ismade of sufficiently flexible material to bulge outward at the pistonflange with the piston flange and retaining cup being dimensioned toextend outward a distance sufficient to form a fluid tight seal withwalls of the filter vial during use.

In further variations, the method includes providing a filter support onthe distal end of the retaining cup, the cup filter support located toabut the filter during use. The method can also include providing apiston filter support on the distal end of the piston, with the pistonfilter support located to abut the filter during use. Advantageously,the method also includes providing a piston filter support on the distalend of the piston, the piston filter support located to abut the filterduring use. Finally, the method includes placing a rib on a distal edgeof the piston located to form an axial facing seal around a periphery ofthe filter.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the various embodimentsdisclosed herein will be better understood with respect to the followingdescription and drawings, in which like numbers refer to like partsthroughout, and in which:

FIG. 1 is an exploded side view of a piston, retainer and vial;

FIG. 2 is a sectional view of FIG. 1;

FIG. 3 is a cross-sectional view of the piston and retainer in anassembled configuration;

FIG. 4 is a cross-sectional view of the piston, retainer and vial in anassembled configuration with the piston partially inserted into thevial;

FIG. 5 is an end view of the retainer cup of FIG. 1, looking from thedistal end toward the proximal end;

FIG. 6 is an end view of the piston of FIG. 1, looking from the distalend toward the proximal end;

DETAILED DESCRIPTION

Referring to FIGS. 1-4, a tubular piston 10 with a circularcross-section has proximal and distal ends 12, 14, respectively, with aporous piston support 16 located at the distal end 14. The support 16advantageously takes the form of radial arms intersecting at a centrallocation on the longitudinal axis 18 of the piston 10. The terms distaland proximal refer to relative locations of parts along the longitudinalaxis 18, and the terms inward and outward refer to relative directionstoward and away from the longitudinal axis 18. One or more flanges 20extend radially from the proximal end 12. A first, proximal seal 22 islocated adjacent to but inward from the proximal end 12 and extendsoutward, away from longitudinal axis 18. A vent hole 24 is adjacent theseal 22 but located slightly toward the distal end 14. Adjacent to andpreferably extending to the distal end 14 is a recessed area 26 having asmaller diameter than that of the body of piston 10 forming a shoulder27 at the location of the larger diameter portion of the piston body. Anoutward extending distal piston flange 28 is located adjacent the distalend 14 in this recessed area. An axially extending tip 31 (FIG. 2) ispreferably, but optionally provided that encircles the distal end of thepiston 10. The tip 31 preferably has a triangular cross section and isvery small.

A retainer cup 30 has an inner diameter sized to fit snugly against thepiston recess 26. The retainer cup 30 has proximal and distal ends 32,34, respectively, with a porous cup support 36 over the distal end 34and a third, cup seal 38 at the proximal end, extending outward, awayfrom the axis 18. The support 36 is like the support 16. A membranefilter 40 fits inside the retainer cup 30 and abuts the cup support 36.The membrane filter 40 is disc shaped, usually with an axial thicknessthat is several times smaller than its diameter. In some situationsmultiple filters 40 can be stacked. An inward extending lip 41 at thedistal end 34 of the cup 30 provides an axial support around theperiphery of the filter 40. The tip 31 (FIG. 2) on the distal end of thepiston 10 presses the filter 40 against this lip 41 when the parts areassembled.

The supports 16, 36 preferably take the form of radial struts or spokesextending from the walls of piston 10 or cup 30, respectively. But thesupports 16, 36 could take other forms, such a rectangular or squaregrid work, or a plate with drain holes therein. A spoke with atriangular cross-section is believed suitable, with the apex of thetriangle abutting the filter 40 in order to reduce blockage of thefilter. Since supports 16, 36 abut opposing sides of the filter 40, aspoke with a triangular cross section will require orientating theapexes so they are toward each other. While four spokes are shown, sixequally spaced spokes are believed preferable. Too many spokes blockflow through the filter 40, and too few spokes allow the filter to flexand allow unfiltered fluid to bypass the filter by flowing around theperiphery of the filter. The intersection of the spokes at the center ispreferably small so as not to block fluid flow through the filter, witha center obstruction of about 0.1 inches for a piston 10 with an innerdiameter of about 0.3 inches. The dimensions will vary with the sizesand with the strength of the materials used.

A tubular vial 42 has an open proximal end 44 and a closed distal end46. The vial 42 is sized and shaped so the retainer cup 30 and piston 10fit within the vial. The vial 42, cup 30 and piston 10 are preferablyall cylindrical with a circular cross section, but other shapes could beused.

Referring to FIG. 3, the retaining cup 30 fits over piston flange 28 andabuts the recessed portion 26 on distal end 14 of piston 10 so thepiston support 16 abuts membrane filter 40 and preferably presses thefilter 40 against lip 41 and against cup support 36. Ideally thecircular tip 31 provides a concentrated force around the periphery ofthe filter 40 to clamp the filter against the lip. The filter 40 is thusheld in position by the lip 41 and tip 31, and further supported by thesupports 16, 36. The tip 31 and lip 41 hold the periphery of the filter40 in position with the tip 31 providing an axial compression sealagainst the filter 40. Movement, such as bowing or twisting of the bodyof the filter 40, is restrained by the supports 16, 36. The combinationof supports 16, 36, lip 41 and tip 31 are sufficient so that the sealbetween the periphery of the filter 40 and lip 41 does not leak asfiltrate fluid 50 is forced through the filter 40.

As the retainer cup 30 fits over the recessed portion 26, the flange 28forces the sidewall 29 of the cup 30 outward to form a fluid tight sealand the proximal end 32 preferably abuts or comes close to shoulder 27.The piston flange 28 is preferably at or very close to the distal end 14of piston 10, and is interposed between the outer wall of piston 10 andthe inner wall of the cup 30 to form a fluid tight seal. Advantageously,the piston support 16 presses the filter 40 against the lip 41 enough toprevent any leakage past the end of the filter 30, and preferably thefilter is compressed by the circular periphery of distal end 14 ofpiston 10, or compressed by the circular periphery of the support 16, sothat any fluid wicking between the outer periphery of the filter 10 andthe wall 29 of the cup 30, does not enter the cup. The location of theshoulder 27 and the axial length of the sidewall 29 of cup 30 can beselected to achieve a desired amount of compression.

As the retainer cup 30 fits over the recessed portion 26 and the seal26, the sidewall 29 of the cup 30 is deformed outward so the outerdiameter of the assembled cup 30 and piston 10 is slightly larger thanthe diameter of the piston body 10. The assembled cup 30 and piston 10are then fit into vial 42 to force fluid or filtrate 50 through thefilter 40 and into the hollow interior of the tubular piston 10.Preferably, but optionally, the piston flange 28 bulges the sidewall 29of cup 30 outward enough to form a fluid tight seal with the inner wallsof vial 42. The seal 38 is also sized relative to the inner walls ofvial 42 to form a fluid tight seal with the vial. Advantageously, thereare thus two outward facing seals adjacent the distal end 14 of thepiston 10 which seal against the interior walls of vial 42.

As the piston is advanced along longitudinal axis 18, fluid or filtrate50 passes though filter 40 and into the body of piston 10, with airescaping through vent 24. Eventually the seal 22 enters the vial 42 andalso seals against the inner walls of vial 42. Filtrate 50 passingthrough the filter 40 is in the hollow body of piston 10 where it can beremoved directly, or through an opening in cap 52 which is fitted overthe proximal end 12 of the piston 10. Materials entrained in thefiltrate 50 which are too large to fit through the filter 40 areretained between the filter 40 and the closed distal end 46 of the vial.Preferably the cap 52 forms a snap fit with the proximal end of thepiston 10, and more preferably has an inwardly extending rim or lip thatforms a snap-fit over flange(s) 20 on the piston 10.

The amount of deformation or bulging caused by forcing the cup 30 overthe flange 28 affects the quality of the fluid tight seal formed betweenthe sidewall 29 of the cup 30 and the inner wall of vial 42 at thelocation of the flange 28. The deformation is affected mostly by thestiffness of the sidewall 29 on cup 30, the thickness of the wall on cup30, the stiffness of the flange 28, the shape and size of the flange 28.Further, the flange 28 is stiff enough that it does not loose itscircular shape and forms a fluid tight seal around the entirecircumference of the abutting inner side of sidewall 29 of cup 30. InFIG. 4, the walls of the vial 42 are shown as slightly deforming at thetwo seal locations, but such deformation is optional and will vary withthe thickness and material used for the vial 42. Advantageously theflange 28 has an inclined surface angled outward and inclining from thedistal toward the proximal end of the piston.

The seal 38 also preferably has an inclined surface angled outward andinclining from the distal toward the proximal end of the piston. Theseal 22 also preferably has an inclined surface angled outward andinclining from the distal toward the proximal end of the piston.

The seals 22, 28 and flange 38 preferably have a generally triangularcross-section, and are integrally molded with the piston 10, or cup 30.This construction allows the walls of piston 10 and vial 42 to be thinwhile the seals 22, 28 and flange 38 are strong and maintain theircircular shape and sealing uniformity during use. Differentcross-sectional shapes could be used. Likewise, the supports 16, 36 areintegrally molded during formation of the piston 10 and cup 30,respectively.

The plug inserted into the hollow body of piston 10 and the hollow cupportion of cup 30 passes along the longitudinal axis 18. If the supports16, 36 each have a triangular cross-section with the apexes of thetriangles facing opposing sides of the filter 40 during use, then amating plug must be used to form the distal end of the piston 10 to formthe triangular cross-section of the supports. Alternatively, thecross-section of the struts could be generally square or rectangular nwhich case the reverse shapes could be formed in the molding plug withonly sufficient taper to allow withdrawal of the plug after molding. Theformation of supports 36 with a triangular cross section on the cup isnot difficult as the triangular shape can be formed in the end of themolding plug and the angles facilitate withdrawal of the plug.

Other seal types could also be used for seals 28, 22, 38, but arebelieved much less desirable. For example, O-ring seals set inrectangular or semi-circular recesses could be used. But the recessesused to position such seals reduce the wall thickness and strength ofthe walls, making them undesirable. Thus, the seals 28, 22, 38 arepreferably integrally molded or cast with the parts from which theyextend. Further, the preferred piston 10, cup 30 and vial 42 are moldedout of various plastics and that makes forming suitable recesses to holdthe O-ring or even D-ring seals difficult because the recesses reducethe strength of the parts.

The piston 10, cup 30 and vial 42 are preferably molded of a suitableplastic, preferably one that does not contaminate samples placed in thevial 42. The vial is preferably made of polyolefin, preferablypolypropylene, or other suitable polymer. The filter 40 is preferably,but optionally made of Teflon, nylon, glass fiber or other filtermaterials such as PVDF (polyvinyldifloride) or PES (polyethersulphone),etc.

In use, the filter 40 is placed inside the retaining cup 30. The cup 30is then snap fit over piston flange 28 and onto the distal end of thepiston 10, and preferably (but optionally) pushed along the piston untilthe cup seal 38 abuts the shoulder 27 on the piston. At this point thefilter 40 is held between the distal end of the piston 10 and lip 41,and preferably held between the piston support 16 and cup support 36 andlip 41. The tip 31 pressing the filter 40 against lip 41 advantageouslyforms an axial seal encircling the filter 40 near the periphery of thefilter, and further helps prevent the periphery of the filter fromdeforming between the spaces in the supports 16, 36 and thus helpsprovide a good seal at the periphery of the filter. The supports 16, 36prevent the filter 40 from bending along axis 18 and breaking the sealat the periphery of the filter.

The tip 31 preferably has a triangular cross-section with the apexabutting the filter 40. During assembly, the tip 31 is preferably smallenough that it deforms when it presses the filter 40 against the lip 41.A tip 31 having a height of about 0.01 inches is believed suitable whenused on a piston 10 with an inner diameter of about 0.3 inches so thediameter of the tip 31 is slightly smaller. The tip 31 may be viewed asa rib on the circular distal edge of the tubular piston 10.

This assembly also forms two outwardly extending portions, one at thelocation where the piston flange 28 forces the sidewall 29 of cup 30outward, and the other at the location of cup seal 38. Both of theseseal locations are larger than the inside of tubular vial 42 and formtwo seals with the vial. Both of these seal locations are preferablyseparated by a distance sufficient that the wall of the vial 42 extendsinward toward longitudinal axis 18 a slight distance in order to ensurea good fluid-tight seal at both locations.

As seen in FIGS. 1-2, the distal piston flange 28 is located slightlybefore the end of the piston 10, so that there is a short cylindricalsection before the inclined flange 28 begins. This cylindrical sectionis optional. This cylindrical section could be formed by ultrasonicallywelding a disc-shaped filter support 16 having a cylindrical peripheryonto the distal edge of the piston 10. But such ultrasonic weldingleaves contaminants, as do adhesives. Thus, the filter support 16 ispreferably integrally molded simultaneously with the formation of thepiston 10 and thus made of the same material as the piston 10. Thisintegral molding is usually difficult because of the length of thepiston 10 is typically several times longer than the diameters of thepiston so it is difficult to mold interior parts.

The piston flange 28, and to a lesser extent the flange 28 and recess26, thus forms a snap lock to hold the retaining cup 30 onto the end ofthe piston 10. But the flange 28 also cooperates with the deformablewall 29 of the retainer cup 30 to bulge the sidewall 29 outward and forma sealing location at the distal end of the cup 30 and at the distal endof the piston 10.

A fill line 54 is optionally placed on the outside of the vial 42 toindicate a maximum level of fluid placed inside the vial. The walls ofthe vial are preferably sufficiently transparent or translucent so theplacement of fluid inside the vial 42 can be seen from outside the vialto allow use of the fill line 54. The fill line 54 can be molded intothe vial as an outwardly extending flange or inwardly extending recess,or the fill line can be marked by paint, marker, abrasion, laseretching, chemical etching, or other process leaving a visible indicia.

The distal end 14 of the piston 10 could hold the filter against theinwardly extending lip 41 on the cup. But preferably one, and optimallyboth supports 16, 36 are also used to hold the filter in place and tosupport the middle of the filter during use. The piston support 16restrains the filter from bowing and leaking around the periphery as thepiston advances into the vial and fluid is forced through the filter.The piston support 16 is thus preferably stronger than the cup support36. The piston 10 can be made of an equal or stronger and hardermaterial than the retaining cup 30, especially since the retaining cup30 must have a sidewall 29 flexible enough to slide over the pistonflange 28 and form a snap fit with the piston recess 26. Preferably thepiston 10 is of the same material as the retaining cup 30, but the cupis slightly thinner and thus more flexible,

The above description is given by way of example, and not limitation.Given the above disclosure, one skilled in the art could devisevariations that are within the scope and spirit of the inventiondisclosed herein, including various ways of shaping the seals 28, 38.Further, the various features of the embodiments disclosed herein can beused alone, or in varying combinations with each other and are notintended to be limited to the specific combination described herein.Thus, the scope of the claims is not to be limited by the illustratedembodiments.

1. A filter vial kit, comprising: a tubular vial having longitudinalaxis, a closed bottom, an open top and a cylindrical interior wall witha diameter D; a tubular piston with opposing distal and proximal endswith an opening in the distal end, the piston having a tubular walldefining an interior, the tubular wall having an outer diameter lessthan D, the piston further having a distal piston flange at or adjacentto the distal end of the piston and extending outward from the piston; afilter support at the distal end of the piston having openings throughwhich fluid can flow into the interior of the piston during use; aretainer cup having a bottom and continuous sidewall with the sidewallextending between the bottom at a distal end of the cup and a proximalend of the cup, the cup sidewall having an interior diametersufficiently smaller than the distal piston flange to allow the sidewallto pass over the distal piston flange, the bottom having an openingthrough which fluid can flow during use, the cup further having a cupseal extending outward from the proximal end of the cup a distancegreater than the diameter D, the sidewall of the cup being sufficientlyflexible to deform outward as the cup passes over the distal pistonflange so the sidewall can have an outer dimension larger than diameterD at the location of the piston flange when the retainer cup passes overthe distal piston flange; and a filter that is configured to fit betweenand be held between the retaining cup and the distal end of the pistonwhen the proximal end of the retainer cup is placed over the distal endof the piston.
 2. The filter vial kit of claim 1, wherein the filtersupport is inside the piston.
 3. The filter vial kit of claim 1, whereinthe cup has a lip extending inward toward the longitudinal axis at thedistal end of the retainer cup a distance sufficient to stop passage ofthe filter along the longitudinal axis.
 4. The filter vial kit of claim1, wherein the cup has a lip encircling a periphery of the opening inthe cup, the lip extending inward toward the longitudinal axis adistance sufficient to stop passage of the filter along the longitudinalaxis.
 5. The filter vial kit of claim 1, wherein a lip on the cupextends continuously around an inner circumference of the retainer cup.6. The filter vial kit of claim 1, wherein the distal end of the pistonhas an outward facing wall with a recessed area therein extendingentirely around a circumference of the piston.
 7. The filter vial kit ofclaim 6, wherein the cup is dimensioned relative to the recessed area sothe cup seal is located in the recessed area when the cup is placed overthe distal end of the piston for use.
 8. filter vial kit of claim 1,wherein the piston, vial and retaining cup are made of a polymer.
 9. Thefilter vial kit of claim 1, further comprising a cap sized to releasablybut snugly fit over the proximal end of the piston.
 10. The filter vialkit of claim 1, further comprising a seal extending outward from thepiston adjacent the proximal end of the piston and located to engage andform a fluid tight seal against the interior wall of the vial when thedistal end of the piston is placed inside of the vial.
 11. The filtervial kit of claim 1, further comprising a seal extending outward fromthe piston adjacent the proximal end of the piston and located to engageand form a fluid tight seal against the interior wall of the vial whenthe distal end of the piston is placed inside of the vial and adjacentthe bottom of the vial.
 12. The filter vial kit of claim 1, wherein thecup is placed on the end of the distal end of the piston with the filterbetween the distal end of the piston and the distal end of the cup. 13.The filter vial kit of claim 1, further comprising an air vent in thevial and located closer to the open top than to the closed bottom of thevial.
 14. The filter vial kit of claim 1, wherein the cup has a cupfilter support with the filter supports of the cup and piston abuttingthe filter when the distal end of the piston is inserted into the cupfor use.
 15. The filter vial kit of claim 14, wherein the filtersupports include radial spokes.
 16. The filter vial kit of claim 1,wherein the piston has an inner diameter of about 0.3 inches.
 17. Afilter vial kit, comprising: a tubular vial having longitudinal axis, aclosed bottom, an open top and a cylindrical interior wall joining thebottom, the interior wall having a diameter D; a tubular piston bodywith opposing distal and proximal ends with an opening in the distalend, the piston having a tubular wall defining an interior of thepiston, the piston further having a distal piston flange at or adjacentto the distal end of the piston and extending outward from the piston,the tubular wall having an outer diameter less than D; a filter supportat the distal end of the piston and having openings through which fluidcan flow into the interior of the piston during use; a retainer cuphaving a bottom and continuous sidewall with a cylindrical innersurface, the sidewall extending between the bottom at a distal end ofthe cup and a proximal end of the cup, the cup sidewall having aninterior diameter sufficiently smaller than the distal piston flange toallow the sidewall to pass over the distal piston flange, the bottomhaving an opening through which fluid can flow during use, the cupfurther having a cup seal extending outward from the proximal end of thecup a distance greater than the diameter D, the sidewall of the cupbeing sufficiently flexible to deform outward as the cup passes over thedistal piston flange so the sidewall can have an outer dimension largerthan diameter D at the location of the piston flange when the retainercup passes over the distal piston flange; and a filter sized to fitinside the cup.
 18. The filter vial kit of claim 17, further comprisinga seal extending outward from the piston adjacent the proximal end ofthe piston and located to engage and form a fluid tight seal against theinterior wall of the vial when the distal end of the piston is placedinside of the vial.